1. Field of the Invention
This invention relates to a developing apparatus used in an image forming apparatus such as a copying apparatus or a printer using the electrophotographic system or the electrostatic recording system to develop an electrostatic image on an image bearing member.
2. Related Background Art
Various apparatuses have heretofore been proposed and put into practical use as electrophotographic developing apparatuses. These developing apparatuses are divided roughly into developing apparatuses using a one-component developing system and developing apparatuses using a two-component developing system. In the one-component developing system, almost all of the apparatuses adopt a noncontact system, and as a typical developing method, there is a one-component jumping developing method using a magnetic toner. This developing method can provide images of high quality by an easy construction, but suffers from the disadvantage that color images cannot be provided because a magnetic material is contained in the toner. Also, a one-component developing method using a nonmagnetic toner can provide color images, but it is difficult to apply the toner onto a developing sleeve and at present, coating is effected by an elastic blade, and this method lacks stability and durability.
On the other hand, the two-component developing method conveys a toner to a developing area by a magnetic carrier and effects development, and usually effects the developing step with the developer brought into contact with a photosensitive drum. Here, the developing step will be described with reference to FIG. 4 of the accompanying drawings. As shown in FIG. 4, the developing apparatus 40 of this example is provided with a developing container 34, a developing sleeve 30 which is a developer carrying (bearing) member opposed to an electrophotographic photosensitive member 50 which is an image bearing member and disposed in the opening portion of the developing container 34, a magnet roller 35 which is magnetic field producing means fixedly disposed in the developing sleeve 30, a regulating blade 33 which is a developer layer thickness regulating member for regulating the layer thickness of a developer carried on the developing sleeve 30, and agitating screws 31 and 32 contained in the developing container 34.
A description will hereinafter be made of the developing step of visualizing an electrostatic latent image formed on the electrophotographic photosensitive member 50 by a two-component magnetic brush method by the use of the above-described developing device 40, and a circulating system for the developer.
First, the developer drawn up by a pole N3 with the rotation of the developing sleeve 30 is regulated by the regulating blade 33 in the process of being conveyed from pole S2 to pole N1, and is formed into a thin layer on the developing sleeve 30. When the developer formed into a thin layer is conveyed to a main developing pole S1, the erection of the developer is formed by a magnetic force. The above-mentioned electrostatic latent image is developed by the developer formed into an ear-like shape, whereafter the developer on the developing sleeve 30 is returned into the developing container 34 by the repulsive magnetic fields of a pole N3 and a pole N2.
A DC bias and an AC bias are applied from a voltage source, not shown, to the developing sleeve 30. Generally, in the two-component developing method, when an AC bias is applied, the developing efficiency increases and the resultant image becomes high in quality, but there arises the danger that fogging is liable to occur.
As a latent image forming method, there is known a method of scanning and exposing an electrophotographic photosensitive member by a laser beam modulated correspondingly to a recorded image signal, and forming an electrostatic latent image comprising latent images of a dot-distributed shape, i.e., a dot-like shape, distributed correspondingly to the image. Above all, the so-called pulse width modulation (PWM) method of modulating the width (i.e., duration time length) of the driving pulse current of a laser correspondingly to the light and shade of a recorded image can provide high recording density (i.e., high resolution) and high gradation property.
In the image forming method using the two-component developing system as described above, to provide a still higher quality of image and a longer life, a developing method using a carrier of low magnetic permeability, or in other words, a carrier having a low magnetization value in a magnetic field of 1000 gauss, is proposed, for example, in Japanese Laid-Open Patent Application No. 6-19222.
By using a carrier of low magnetic permeability, the carrier (developer) is weakly restrained on the developing sleeve, whereby the frictional sliding force of a magnetic brush with respect to a toner image developed for the electrostatic latent image on the photosensitive drum in a developing portion is weakened, whereby a higher quality of image becomes possible. Also, the packing pressure of a developer reservoir upstream of the developer layer thickness regulating blade with respect to the direction of rotation of the developing sleeve drops and the deterioration of the toner is reduced, whereby a longer life becomes possible.
In that case, assuming that the value of magnetization in a magnetic field of 1000 gauss is used as the standard of the magnetization or magnetic permeability of the carrier, when use is made of a low magnetic permeability carrier having magnetization of 200 emu/cm.sup.3 or less in the magnetic field of 1000 gauss, particularly the deterioration of the toner described above is reduced. Also, at this time, it is minimally necessary from the viewpoint of stable coating on the developing sleeve to have magnetization of 30 emu/cm.sup.3 or greater.
On the other hand, the use of the low magnetic permeability carrier leads to the problems of carrier adherence and low image density in a high density portion. Carrier adherence occurs due to the fact that the magnetization of the carrier is small, whereby in a nonimage portion, an electrostatic force by fog-removing bias Vback is superior to a magnetic force. This is due to the fact that the fog-removing bias Vback is an electric field in a direction to pull, the fog toner back to the developing sleeve from the photosensitive drum and the carrier opposite in polarity to the toner becomes an electric field conversely attracting the toner toward the photosensitive drum.
As the causes of the carrier adherence, there are conceivable, besides the case as described above where the electrostatic force has become stronger than the magnetic force, a case where charges have been poured into the carrier, and a case where the carrier adheres to the photosensitive drum by the mirror image force by the poured charges. The pouring of charges into the carrier is due chiefly to the AC component of the developing bias.
On the other hand, low image density occurs particularly in a high density portion because the magnetization of the carrier is small, whereby the length of the magnetic brush becomes short and in a developing area, the length of the developer contacting with the photosensitive drum in the circumferential direction of the photosensitive drum becomes short and the developer assumes its noncontact state particularly when the amount of coat of the developer on the developing sleeve is small and the gap between the developing sleeve and the photosensitive drum (hereinafter referred to as the S-Dgap) is wide. Also, at this time, the so-called edge emphasis that the edge portion of the high density portion is emphasized becomes liable to occur.
Of the aforedescribed problems arising from the use of the low magnetic permeability carrier for a high quality of image and a long life, the adherence of the carrier can be greatly reduced by making the resistance of the carrier great (for example, Japanese Patent Application Laid-Open Nos. 8-227225 and 8-160671). In that case, as the specific resistance of the carrier, the use of a magnetic carrier having 10.sup.12 .OMEGA.cm or greater at electric field intensity of 5.times.10.sup.4 V/m is effective. However, making the resistance of the carrier greater reduces the opposed electrode effect to the charges on the photosensitive drum and therefore, the above-mentioned edge emphasis is promoted.
Also, of the aforedescribed problems, regarding the low image density in the high density portion and the edge emphasis, a great effect is obtained by making the direction of movement of the photosensitive drum and the direction of movement of the two-component developer opposite to each other in the developing area (counter phenomenon). However, when the amount of coating of the developer on the developing sleeve is small and the S-Dgap is wide, the developer becomes more approximate to the noncontact state and the low image density in the high density portion and the edge emphasis become somewhat liable to occur.